The US state department has issued a global travel alert because of an unspecified al-Qaeda threat.
In a statement, the department said the potential for an attack was particularly strong in the Middle East and North Africa.
It comes shortly after the US announced nearly two dozen embassies and consulates would be shut on Sunday.
The alert expires on 31 August 2013, the department said. It recommended US citizens travelling abroad be vigilant.
It's not unusual for the state department to issue travel
warnings, ranging from a caution to an alert. But this one
is both wide-ranging and somewhat specific, as it comes
after the announcement that 14 American embassies will
be closed in the Middle East and North Africa.
The alert is based on intelligence that goes beyond the
usual chatter, according to lawmakers, but the state
department is also acting out of an abundance caution.
Last year, the US issued a worldwide warning about
potential violence ahead of the anniversary of 11
September 2001 attacks. Violent protests against an
anti-Muslim video also erupted in Cairo and Tunis. Then,
in Benghazi, on 11 September, the US ambassador to
Libya was killed in an attack. In the aftermath, US officials
said there was no "actionable" intelligence about an
"Current information suggests that al-Qa'ida and affiliated organizations continue to plan terrorist attacks both in the region and beyond, and that they may focus efforts to conduct attacks in the period between now and the end of August," the statement said.
The alert warned of "the potential for terrorists to attack public transportation systems and other tourist infrastructure".
'Not chit chat'
Nancy Pelosi, the Democratic leader in the House of Representatives, said that congressional leaders had been briefed about the alert.
"There is some understanding of the seriousness of the threat," she told reporters.
Republican lawmaker Jason Chaffetz said he understood there was "a very real worldwide threat".
Dutch Ruppersberger, the top Democrat on the House intelligence committee, said the threat was not prompted by "the regular chit chat" gleaned from would-be militants online or elsewhere.
"The most important thing we have to do is protect American lives," he told the Associated Press news agency.
An unnamed senior US official told NBC the threat may be related to the Islamic holy month of Ramadan, which ends next week.
In the Muslim world, Sunday is a work day. In other parts of the world US diplomatic offices are shut on Sunday.
"It is possible we may have additional days of closing as well," a senior state department official told the BBC on Thursday.
Four Americans died in an attack last year on the US
consulate in Benghazi, Libya
The official said the state department had "been apprised of information" leading it to take these "precautionary steps... out of an abundance of caution".
Last year on 11 September, the US consulate in Benghazi, Libya, was attacked, leaving four Americans dead.
Other embassies are routinely targets of protesters.
The US diplomatic missions to be closed on Sunday are in Abu Dhabi, United Arab Emirates; Algiers, Algeria; Amman, Jordan; Baghdad, Iraq; Cairo, Egypt; Dhahran, Saudi Arabia; Djibouti, Djibouti; Dhaka, Bangladesh; Doha, Qatar; Dubai, United Arab Emirates; Erbil, Iraq; Jeddah, Saudi Arabia; Kabul, Afghanistan; Khartoum, Sudan; Kuwait City, Kuwait; Manama, Bahrain; Muscat, Oman; Nouakchott, Mauritania; Riyadh, Saudi Arabia; Sanaa, Yemen and Tripoli, Libya.